"Blue Moon" to Shine on New Year's Eve

For the first time in almost 20 years, a bright "blue moon" will grace New Year's Eve celebrations worldwide. (Take a moon myths quiz.)

If the skies are clear, revelers looking up at midnight will get an eyeful of the second full moon of the month—commonly called a blue moon. The last time a blue moon appeared on New Year's Eve was in 1990, and it won't happen again until 2028.

A blue moon isn't actually blue—as commonly defined, the name reflects the relative rarity of two full moons in a month and is linked to the saying "once in a blue moon."

With this New Year's Eve blue moon, "there is nothing scientific about it, and it has no astronomical significance," said Mark Hammergren, a staff astronomer at the Adler Planetarium in Chicago, Illinois.

"But I believe it does give us some insight into history and makes us think of how our calendar system has derived from motions of objects in the sky."

Blue Moon Error

The popular definition of a blue moon isn't the only one—and it's one that's based on an editorial error, astronomers contend.

The widespread definition of the second full moon in a month stems from errors made in an astronomy magazine, when a writer misinterpreted how the term was used in the Maine Farmer's Almanac.

Later studies of almanacs published from 1819 to 1962 revealed that the term "blue moon" actually refers to the "extra" full moon that can occur in a year due to differences between the calendar year and the astronomical year.

Most years on average have 12 full moons, with 1 appearing each month.

That's because the lunar month—the time it takes the moon to cycle through its phases—corresponds closely to the calendar month.

But the calendar year is actually based on the solar cycle, or the time it takes Earth to make one trip around the sun. This means a year is not evenly divisible by lunar months, so every three years or so there are 13 full moons.

The farmer's almanac further divided the year into four seasons, with each season lasting three months. When a given season saw four full moons, the almanac dubbed the third moon as a blue moon.

Ultimately, a blue moon as defined by the calendar isn't that rare, added Hammergren. The term's significance instead lies in the way it links people to the motions of the cosmos.

"Just being able to recognize that we can have a full moon twice in a month and have [folklore] attached really highlights the fact that humans have been astronomers their entire existence," he said.

"True" Blue Moon

Before the editorial error, the term "blue moon" more often referred to the rare instances when the moon actually seemed to turn blue, as can happen under certain atmospheric conditions.

"After a forest fire or volcanic eruption, there may be enough particulate matter in the air so that the moon can take on a bluish tinge," Hammergren said.

For instance, a "true" blue moon occurred in 1950 after a large forest fire in Canada blew smoke across most of the Northern Hemisphere.

Another appeared in 1980 after the last major eruption of Mount St. Helens, which sent tons of ash into the upper atmosphere.

Even if the 2009 New Year's Eve blue moon has astronomers scoffing, nighttime partygoers may still get moonstruck.

Rising in the east at sunset, the New Year's Eve full moon will reach its highest point at midnight, noted Jack Horkheimer, director of the Miami Space-Transit Planetarium and host of PBS television's long-running show Star Gazer.